Maintaining a traffic connection of a moving subscriber is made possible by means of handover function. The basic concept of handover is simple: when a mobile station (such as a mobile phone, a wireless Internet device, a portable device, a personal digital assistant, etc.) of the subscriber moves in a cellular communication system from one cell of the system to another, a new connection with the new cell has to be set up and the connection with the old cell has to be released.
Depending on the diversity used the handover mechanisms can be categorized as hard handover, soft handover and softer handover. A simultaneous soft and softer handover process is called soft-softer handover. All these are typically provided by the third generation (3G) mobile systems.
Another way to categorize various handover types is based on system architecture, meaning the network elements between which handover signalling is executed. For instance, if handover is executed between two base stations then it is called inter-base station handover. Based on this criterion, intra-base station, inter-base station, intra-radio access controller, inter-radio access controller and inter core network handover can also be defined, for example. Further examples are intra-frequency handover, which is executed between different layers of the same network or neighbouring cells, inter-frequency handover, which is executed between different cells or layers of the same network or different networks/network types, and intersystem handover, which is executed between cells or layers of different network types, for example IP RAN (IP Radio Access Network), UTRAN (UMTS Radio Access Network) and GSM. IP RAN is an access network which utilizes IP transport. IP transport may be utilized on top of different kind of radio access techniques (multiradio).
On the other hand, depending on the reason on the basis of which the handover decision is made, the handover types can be categorized as traffic reason handovers, quality based handovers, speed based handovers, and so on.
Both second (a second generation system like the GSM will support high rate data traffic in its next phases e.g. with EDGE evolution) and third generation cellular systems provide a remarkable increase in the data bit rate i.e. the transfer rate with which data is transmitted over an air interface. As a result, it can be foreseen that databased services provided to an end user will form a vital part of cellular technology and business in the near future. It is commonly understood that databased services will play a dominant part in the cellular system environments in comparison to real time services such as voice. As a potential business factor, databased services with different Quality of Services (QoS) also set new technical requirements to the cellular systems supporting data communication.
In a real cellular environment the user density and the data bit rate requested by the users are two crucial factors when efficiently planning the cellular system. However, these are some of the most difficult factors to be estimated or controlled, requiring continuous adaptation of the network to the on demand traffic. In case of databased services, it is not enough to adapt the network resources only by means of initial network replanning. Moreover, the nature of the databased services requires more flexibility from other mechanisms like the handover mechanism supported in a cellular system for utilizing the optimized resources to meet the appropriate quality of services.
Although conventional handover mechanisms may be necessary to be supported, they do not always provide means to achieve an optimum performance in the databased service environment.